Conventional heating, warm water and steam boilers which use oil are badly adaptable to solid fuel. Different pre-burner constructions are known, but they have several disadvantages. Full effect is normally not achieved, because the properties of fuel gases differ considerably from the measured values. The combustion gas temperature of a pre-burner that burns moist fuel stays much lower than that of oil heating which has been used as the standard measure. The share of water vapor is considerably greater than that measured. The situation will improve essentially if the moist fuel is dried with combustion gas before it is burnt. Thereby, the combustion temperature and together with it the combustion capacity will rise. However, no practical and economical way to convert a 0.5-10 MW oil boiler of low effect to a boiler with solid fuel has been found. It is difficult to make a fuel dryer work at the end of a conventional oil boiler, because the final temperature of combustion gas is too low. The aforementioned pre-burner solutions are quite awkward and expensive. The efficiency usually stays low and the greatest combustion capacity remains much lower than the nominal capacity of an oil boiler. Dust combustion does not normally come into question because it is so expensive.